Remove extra slot for metadata

[sritchie]

TODO: decide if this actually works in all cases for the affected types. Am I always hanging metadata off something that can take it?

[codecov]

Codecov Report

Patch coverage: 53.70% and project coverage change: -0.11% ⚠️

Comparison is base (e16b569) 86.05% compared to head (4320644) 85.95%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #147      +/-   ##
==========================================
- Coverage   86.05%   85.95%   -0.11%     
==========================================
  Files          98       98              
  Lines       15335    15295      -40     
  Branches      834      827       -7     
==========================================
- Hits        13197    13147      -50     
- Misses       1304     1321      +17     
+ Partials      834      827       -7     

Files Changed	Coverage Δ
src/emmy/structure.cljc	88.04% <45.71%> (-0.04%)	⬇️
src/emmy/series.cljc	59.01% <46.23%> (-2.78%)	⬇️
src/emmy/operator.cljc	76.08% <72.72%> (-1.65%)	⬇️
src/emmy/polynomial.cljc	83.28% <100.00%> (ø)

... and 6 files with indirect coverage changes

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